Effect of hypotonic expansion on sodium, water, and urea excretion in late pregnancy: the influence of posture on these results

Mechanisms of Renal Vasodilation and Hyperfiltration During Pregnancy

Glomerular filtration rate (GFR) and renal plasma flow (RPF) increase by 40-65% and 50-85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration appears to be almost completely due to the increase in RPF, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The major pregnancy hormone involved is relaxin. The mediators downstream from relaxin include endothelin (ET) and nitric oxide (NO). New evidence indicates that relaxin increases vascular gelatinase activity during pregnancy, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and NO. Whether the chronic volume expansion characteristic of pregnancy contributes to the maintenance of gestational renal changes requires further investigation. Additional studies are also needed to further delineate the molecular basis of these mechanisms and, importantly, to investigate whether they apply to women.

Sodium balance in pregnancy

Sodium is the major cation in the extracellular fluid volume (ECFV) and as such, is the most important determinant of osmolality and of the volume of this fluid compartment. Hence any alteration in the control of body sodium will be reflected by changes in the ECFV, including the maternal plasma volume. There is no doubt that expansion of the plasma volume is a necessary and desirable event during pregnancy, influencing positively both maternal and fetal outcome. Therefore, studies of sodium balance in pregnancy provide important information relevant to both mother and fetus.

Renal haemodynamics and tubular function in human pregnancy

In human pregnancy, effective renal plasma flow and glomerular filtration rate increase to levels 50-80% above non-pregnant values. The increments occur shortly after conception, persist throughout the second trimester and reduce slightly in late pregnancy. The hyperfiltration of pregnancy does not seem to be a potentially damaging process, as intraglomerular pressure remains unchanged. The increased excretion of glucose and other nutrients, as well as uric acid and protein, is related in part to altered tubular function. Renal physiology is altered so much in pregnancy that non-pregnant norms cannot be used in antenatal care.

Volume homeostasis in normal pregnancy and pre-eclampsia: physiology and clinical implications

The pieces of the jigsaw puzzle of volume homeostasis in human pregnancy are being put together gradually. This chapter has focused on recent advances in our understanding of factors controlling extracellular fluid volume in normal pregnancy and their disturbance in women who develop pre-eclampsia. We have explored the clinical implications of these guidelines for management of patients with pre-eclampsia. Clearly there is still much to be learned. Studies of the cellular and subcellular handling of sodium are still in their infancy and will add much to our understanding of the physiology of volume homeostasis in normal pregnancy and its disturbance in pre-eclampsia and other causes of hypertension in pregnancy.

Sodium excretion in normal and hypertensive pregnancy: a prospective study

One hundred fifty-eight intravenous saline solution infusions (3 mmol Na per kilogram body weight) were performed in (1) normal primigravid women during the second and third trimesters and post partum, after 1 week of either a high, low, or ad libitum salt intake; (2) normotensive primigravid women during midpregnancy who later developed pregnancy-induced hypertension, and (3) seven proteinuric and seven nonproteinuric primigravid women with ad libitum salt intake who had established pregnancy-induced hypertension. Sodium excretion did not differ significantly between pregnancy and after pregnancy despite marked differences in plasma renin activity, aldosterone concentration, volume, and glomerular filtration rate. Sodium excretion after saline solution loading varied according to prestudy sodium intake and was reduced between the second and third trimesters, independent of dietary salt intake. Those destined to develop pregnancy-induced hypertension had sodium excretion similar to that of continuously normotensive subjects during the second trimester, but those with established proteinuric pregnancy-induced hypertension had the lowest plasma volume, plasma aldosterone concentration, and plasma renin activity and retained sodium to the same degree as salt-deplete women with normotension. These results demonstrate that the balance of sodium regulatory factors is similar between pregnancy and post partum, that prestudy salt intake and stage of gestation can alter the natriuretic response to saline solution loading, and that normal pregnant women retain more administered sodium in late pregnancy than in midpregnancy despite further increases in plasma volume and no alterations to blood pressure or glomerular filtration rate. Those with established proteinuric pregnancy-induced hypertension retain sodium avidly without stimulation of plasma renin activity or plasma aldosterone concentration, findings not apparent during midpregnancy in those who later developed this disorder.

Volume homeostasis in normal and hypertensive human pregnancy

In this chapter are outlined the many factors involved in the regulation of sodium and volume homeostasis in normal human pregnancy and their interrelationships. New developments concerning the role of sodium/potassium ATPase, atrial natriuretic peptide, arginine vasopressin and angiotensin II as regulatory forces are outlined, together with a review of earlier work. Abnormalities found in women with, or destined for, PAH are described and their significance is discussed.

Control of sodium excretion in human pregnancy

Sodium and water retention are normal accompaniments of pregnancy achieved by many significant and complex interrelated alterations in homeostatic forces. Furthermore, potentially harmful derangements in sodium and volume control are found in women with pregnancy-associated hypertension (P-AH, preeclampsia). None of these alterations is completely understood, and many of them appear paradoxical. Recent reviews examined some of the more closely investigated facets of sodium and volume control in normal and hypertensive pregnancy. This relatively brief discussion aims to summarize the known information, update it with more recent work, and speculate on the potential significance of abnormalities in sodium homeostasis in hypertensive pregnancy.

The osmoregulatory system and the renin-angiotensin-aldosterone system in pre-eclampsia and normotensive pregnancy

Plasma concentrations of arginine vasopressin (AVP), angiotensin II (A II), aldosterone (Aldo), serum osmolality (Sosm), urine volume (V), and free water clearance (CH2O) were determined in the third trimester of pregnancy, and 5 days and 3 months after delivery in pre-eclampsia (group I), in normotensive pregnancy (group II), and in non-pregnant control subjects (group III). The AVP was the same in the third trimester of pregnancy in groups I and II and did not deviate significantly from the level in group III. However, 5 days after delivery, AVP was lower and V and CH2O higher in group I than in group II. There was no correlation between AVP and Sosm in the third trimester in either group I or II, but 5 days after delivery a significant positive correlation was found between these parameters in both groups I and II as well as in group III. The A II and Aldo changed qualitatively in the same way in groups I and II, that is, considerable elevation in the third trimester and normalization after delivery. Also, A II and Aldo were lower in group I than in II. The AVP and A II were not correlated and there was no significant relationship between systolic or diastolic blood pressure on the one hand and AVP, A II or Aldo on the other in either group I or II. Thus the osmoregulatory system appears to be altered in both pre-eclampsia and normotensive pregnancy, but becomes normal again 5 days after delivery. In pre-eclampsia a suppression of AVP seems to be responsible for the elimination of excess water in pre-eclampsia 5 days after delivery. There was no evidence for a causal relationship between blood pressure and the osmoregulatory system or the renin-angiotensin-aldosterone system in any of the pregnant groups.

Complications of beta-mimetic therapy in preterm labour

Selective beta-mimetic drugs are the most frequently used agents for arresting preterm labour. For this purpose these drugs have to be given in high doses for several hours and since they act on several organ systems, side-effects, some of which are potentially dangerous, appear. These hazards, which involve chiefly the cardiovascular and metabolic areas, have been examined in the light of available experience with these drugs. This review considers the possible mechanisms for the side-effects and offers suggestions on how the dangers in the use of beta-mimetic drugs in preterm labour might be avoided.

Response of the renin-aldosterone system and antidiuretic hormone to oral water loading and hypertonic saline infusion during and after pregnancy

Amongst the man physiological changes in human pregnancy are the sustained stimulation of the renin-angiotensin system and a decreases in plasma osmolality (Posm). In this study the effect osmolar and water loading on the renin-angiotensin system and arginine vasopressin (AVP) secretion has been tested in seven women during the third trimester and again 8-10 weeks after delivery. Pregnant women had markedly increased plasma renin substrate (PRS), plasma renin activity (PRA) and plasma renin concentration (PRC) values as well as aldosterone levels when compared to their post-partum values. Osmolar loading with intravenous infusion of hypertonic saline resulted in a decrease in PRA and aldosterone levels both during and after pregnancy but even at the end of the infusion the pregnant women still had values greater than the pre-infusion levels obtained post-partum. Surprisingly, oral water loading also significantly decreased PRA and aldosterone levels in pregnancy, possibly related to the redistribution of extracellular fluid centrally when the pregnant women were in left lateral recumbency. Despite the decreased basal Posm of pregnancy, urinary AVP increased and decreased appropriately during osmolar and water loading. Exact characterisation of the resetting of the threshold for AVP secretion in pregnancy awaits the development of a reliable radioimmunoassay for the determination of AVP in human pregnancy plasma.

Osmoregulation during pregnancy in the rat. Evidence for resetting of the threshold for vasopressin secretion during gestation

Osmoregulation was studied in near term and age-matched Sprague-Dawley rats. Basal plasma osmolality (P(osm)) and plasma sodium (P(Na)) were 281+/-3 mosmol/kg and 134+/-3 meq/liter, respectively, on the 20th gestational day compared with 292+/-3 mosmol/kg and 140+/-1 meq/liter in virgin animals (P < 0.001), whereas P(urea) and plasma water content were similar in pregnant and control rats. These differences could not be reproduced in animals receiving progesterone, estrone, or a combination of progesterone and estradiol for 2 wk. Pregnant and control rats were deprived of water for periods ranging from 0 to 48 h. P(osm), always lower in gravidity, was 290+/-3 mosmol/kg after 2 d of water deprivation in pregnant animals compared with 300+/-2 mosmol/kg in controls (P < 0.001). Thus 48 h of dehydration were required before P(osm) in gravid rats was similar to basal values in the age-matched virgins. Despite strikingly lower P(osm), plasma arginine vasopressin (P(AVP)) and urinary osmolality (U(osm)) were similar in the basal state averaging 2.16+/-0.78 pg/ml and 1,652+/-406 mosmol/kg, respectively, during pregnancy compared with 2.08+/-2.17 pg/ml and 1,483+/-203 mosmol/kg in controls (NS). Water deprivation increased P(AVP) and U(osm) similarly in pregnant and virgin rats: these values reached 22.7+/-3.3 pg/ml and 3,300+/-123 mosmol/kg at 48 h in gravid compared with 26.0+/-6.4 pg/ml and 3,342+/-141 mosmol/kg in the controls (NS). Regression equations for P(AVP)vs. P(osm) which were highly significant (P < 0.001) in both groups demonstrated an apparent threshold for AVP secretion approximately 11 mosmol lower in gravid animals. Intravascular volume decreased, and plasma aldosterone increased during water deprivation, and both changes (Delta%) were significantly greater in the gravid animals (P <0.01). Therefore, P(osm) was increased without concomitant volume depletion by intraperitoneal hypertonic saline. Again P(AVP)vs. P(osm) correlated significantly (r > 0.9; P < 0.001) in each group, and the apparent threshold was 14 mosmol lower in pregnant animals. Diluting ability, tested by oral water loading, was not impaired in the pregnant animals which excreted a 30 ml/kg load as well as controls. Also, chronically hydrated virgin animals whose fluid intake was more than twice that of pregnant rats (for 19 d) did not lower their P(osm). In separate studies homozygous Brattleboro rats, which produce no endogenous vasopressin, were also shown to have a decreased P(osm) (pregnant 292+/-4 mosmol/kg; virgin 310+/-6 mosmol/kg P < 0.001), but unchanged U(osm) during pregnancy. Data demonstrate a resetting of the osmostat in gravid Sprague-Dawley rats as P(osm) and the threshold for AVP secretion both decrease significantly during gestation in this species. Studies in homozygous Brattleboro animals with hereditary diabetes insipidus suggest that the osmotic threshold for thirst is reset as well.

Non-postural serial changes in renal function during the third trimester of normal human pregnancy

Seventeen healthy women were investigated near the beginning and again near the end of the third trimester of their normal pregnancies. Infusion studies were performed in the left lateral position. There was a highly significant decrease in effective renal plasma flow but not in glomerular filtration rate, measured as inulin clearance. Plasma creatinine concentration increased significantly, but the renal handling of creatinine was unchanged; simultaneous 24-hour creatinine clearance showed a tendency to decrease. Serum urate concentration also increased significantly, apparently due to an increase in net tubular reabsorption of urate.

Serial changes in renal haemodynamics during normal human pregnancy

Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were determined at constant intervals during and after the normal pregnancies in 25 healthy women. Compared with non-pregnant values, ERPF increased by 80 percent during early pregnancy but fell significantly from this new level during the third trimester. GFR, however, remained at a level 50 per cent above the non-pregnant throughout pregnancy. Filtration fraction (GFR/ERPF) was significantly reduced during early pregnancy but rose to a value equivalent to the non-pregnant during the third trimester. Comparable data of previous workers are re-interpreted.

Renal physiology in pregnancy

The changes which occur in renal excretory function during pregnancy are discussed. The relationship between glomerular filtration rate and effective renal plasma flow is examined in the light of new serial data obtained under conditions of saline diuresis. Changes in the renal handling of nutrients and in the excretion of waste products during pregnancy are reviewed.

Tubular reabsorption of sodium during acute and chronic volume expansion in man

Renal hemodynamics and tubular fractional sodium reabsorption (FSR) were evaluated by clearance techniques during acute and chronic extracellular volume expansion in man. (1 - V/GFR) x 100 was used as an index of proximal and (C(H2O)/V) x 100 as an estimate of distal fractional reabsorption. After acute loading with isotonic saline 37 ml/kg body wt, proximal FSR decreased by 4.8% and distal FSR decreased by 4.4%. After comparable chronic expansion by mineralocorticoids ("escape"), proximal FSR also decreased by 3.9%, but distal reabsorption was not altered.In separate studies, subjects were progressively infused with saline to 57 (E(1)) and to 80 (E(2)) ml/kg body wt, and appeared to divide into "excreters" (maximum U(Na)V > 1000 muEq/min) and "nonexcreters" (maximum U(Na)V < 550 muEq/min). In the excreters, GFR rose, proximal FSR decreased by 7.1% after E(1) and only 0.9% further after E(2). Distal FSR fell by 14.8% after E(1) and by an additional 4.9% after E(2). In the nonexcreters, GFR was stable and proximal FSR did not fall significantly after E(1) or E(2). Distal FSR decreased 4.5% after E(1) and 1.3% further after E(2). It is concluded that both acute and chronic extracellular expansion decrease proximal FSR in man, but only acute loading depresses distal FSR. Ability of some men to excrete sodium rapidly after acute infusion is related to larger increases in GFR and greater decreases in both proximal and distal FSR than occur in men in whom natriuresis is more limited.